Method of controlling a capacitance of a thin film transistor liquid crystal display (TFT-LCD) storage capacitor

ABSTRACT

A method of controlling the capacitance of a thin film transistor liquid crystal display (TFT-LCD) storage capacitor is disclosed. In certain embodiments, the method includes i) forming a silicon island and a bottom electrode on the transparent substrate, the silicon island having an undoped region located on the central portion, and two doped regions respectively located on both sides, ii) forming a first silicon nitride layer on the transparent substrate, and iii) forming a stacked layer comprising a second silicon nitride layer and a conductive layer on the undoped region of the silicon island, and the first conductive layer of the stacked layer serving as a gate of a thin film transistor, wherein an etching selectivity ratio of the conductive layer over the dielectric layer is not less than about 5.0.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/289,470, filed Nov. 6, 2002 now U.S. Pat. No. 6,800,510, the fulldisclosure of which is incorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a thin film transistor liquid crystaldisplay (TFT-LCD). More particularly, the present invention relates to amethod of controlling the capacitance of a TFT-LCD storage capacitor.

2. Description of the Related Art

Liquid crystal display (LCD) has many advantages over other conventionaltypes of displays including high display quality, small volumeoccupation, lightweight, low voltage driven and low power consumption.Hence, LCDs are widely used in small portable televisions, mobiletelephones, video recording units, notebook computers, desktop monitors,projector televisions and so on. Therefore, LCD has gradually replacedthe conventional cathode ray tube (CRT) as a mainstream display unit.

The gate dielectric layer of the thin film transistor in the TFT-LCD isgenerally a silicon nitride layer. When a source/drain stacked layer ina bottom gate structure or a gate in a top gate structure is formed onthe gate dielectric layer, a short period of over-etching is performedto make sure that no residues are left on the gate dielectric layer.Since the area of the transparent substrate is very large, the thicknessuniformity of the gate dielectric layer after over-etching on the entiretransparent substrate is not good. Thus, the thickness uniformity of thestorage capacitor dielectric layer in each pixel is also affected.

The storage electricity of the storage capacitor is used to compensatefor the leakage current of the pixel electrode, and the pixel electrodevoltage can therefore be maintained at a stable level to stabilize thearrangement of liquid crystal molecules to stabilize the display of LCD.If the capacitances of the capacitors on the transparent substrate arevaried, the charging or discharging rates are also varied. Therefore,the TFT dimensions cannot be designed according to the ideal conditionthat each storage capacitor has the same capacitance. To insure that astorage capacitor with less sufficient capacitance can normally chargeand discharge in a regular time period, the TFT dimensions have to bedesigned large enough to enable the poorest storage capacitor tofunction normally. Therefore, the stability of the TFT-LCD display canbe maintained. However, the aperture ratio of each pixel in TFT-LCD isdecreased.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

One aspect of the invention provides a method of controlling thecapacitance of the TFT-LCD storage capacitor to control the uniformityof the storage capacitor's dielectric layer.

Another aspect of the invention provides a method of controlling thecapacitance of the TFT-LCD storage capacitor to improve the uniformityof the storage capacitor's capacitance.

Another aspect of the invention provides a method of controlling thecapacitance of the TFT-LCD storage capacitor to reduce TFTs' dimensions.

Still another aspect of the invention provides a method of controllingthe capacitance of the TFT-LCD storage capacitor to elevate the apertureratio of the liquid crystal display.

In one embodiment, a method of controlling the capacitance of theTFT-LCD storage capacitor is provided. The method comprises thefollowing steps. A first conductive layer is formed on a transparentsubstrate and then is patterned to form a gate and a bottom electrode. Afirst silicon nitride layer, a dielectric layer, a second siliconnitride layer, an undoped amorphous silicon layer, and a doped amorphoussilicon layer are sequentially formed on the transparent substrate, andan etching selectivity ratio of amorphous silicon over a material of thedielectric layer is not less than about 5.0. The doped amorphous siliconlayer, the undoped amorphous silicon layer, and the second siliconnitride layer are patterned to form a stacked layer on the dielectriclayer over the gate. A second conductive layer is formed on thetransparent substrate. Then, the second conductive layer and the dopedamorphous silicon layer are patterned to form a source and a drain oneither side of the gate. Next, a passivation layer is formed over thetransparent substrate and then is patterned to form a contact window toexpose the source or the drain. A transparent conductive layer is formedon the passivation layer and in the contact window. The transparentconductive layer then is patterned to form a pixel electrode to connectthe exposed source or the drain through the contact window electrically,and a storage capacitor is formed by the overlap between the pixelelectrode and the bottom electrode.

In another embodiment, a method of controlling the capacitance of theTFT-LCD storage capacitor is provided. The method comprises thefollowing steps. A first conductive layer is formed on a transparentsubstrate and then is patterned to form a gate and a bottom electrode onthe transparent substrate. A first silicon nitride layer, a dielectriclayer, a second silicon nitride layer, an undoped amorphous siliconlayer, and an etching stop layer are sequentially formed on thetransparent substrate, and an etching selectivity ratio of amorphoussilicon over a material of the dielectric layer is not less than about5.0. The etching stop layer is patterned to form an etching mask on theundoped amorphous silicon layer over the gate. A doped amorphous siliconlayer and a second conductive layer are sequentially formed over thetransparent substrate. Then, the second conductive layer, the dopedamorphous silicon layer, the undoped amorphous silicon layer, and thesecond silicon nitride layer are sequentially patterned to form a sourceand a drain on either side of the gate, and the undoped amorphoussilicon layer serves as a channel between the source and the drain. Apassivation layer is formed over the transparent substrate and then ispatterned to form a contact window therein to expose the source or thedrain. A transparent conductive layer is formed on the passivation layerand in the contact window. Then, the transparent conductive layer ispatterned to form a pixel electrode to connect the exposed source ordrain electrically through the contact window, and a storage capacitoris formed by the overlap between the pixel electrode and the bottomelectrode.

In still another embodiment, a method of controlling the capacitance ofthe TFT-LCD storage capacitor is provided. The method comprises thefollowing steps. An undoped amorphous silicon layer is formed on atransparent substrate and then is patterned to form a silicon island ofthe TFT and a bottom electrode of the storage capacitor on thetransparent substrate. A first silicon nitride layer, a dielectriclayer, a second silicon nitride layer, and a first conductive layer aresequentially formed on the transparent substrate, and an etchingselectivity ratio of amorphous silicon over a material of the dielectriclayer is not less than about 5.0. Then, the first conductive layer andthe second silicon nitride layer are patterned to form a stacked layeron the central part of the silicon island, and the first conductivelayer of the stacked layer serves as a gate of a thin film transistor.The gate is used as a mask to implant ions into the silicon island underboth sides of the gate to form a source and a drain of the thin filmtransistor and implant ions into the bottom electrode. A passivationlayer is formed over the transparent substrate. The passivation layer,the dielectric layer and the first silicon nitride layer then arepatterned to form a first contact window to expose the source and asecond contact window to expose the drain. A second conductive layer isformed over the transparent substrate and then is patterned to form adata line connecting the source through the first contact window. Atransparent conductive layer is formed over the transparent substrate.The transparent conductive layer is patterned to form a pixel electrodeconnecting the drain through the second contact window, and a storagecapacitor is formed by the overlap between the pixel electrode and thebottom electrode.

In various embodiments, the dielectric layer is, for example, a siliconoxide layer, a tantalum oxide layer, an aluminum oxide layer or a bariumtitanate layer.

Various inventive embodiments allow the dielectric layer to be an etchstop layer when the stacked layer is formed, and the remainingdielectric layer and the first silicon nitride layer thus have a uniformthickness. Therefore, the storage capacitor, which comprises overlappingparts of the bottom electrode, the first silicon nitride layer, thedielectric layer, the passivation layer and the pixel electrode, has auniform dielectric layer, which comprises the first silicon nitridelayer, the dielectric layer, and the passivation layer. As a result, thecapacitance of the storage capacitor is also uniform to allow a smallerdimension of the thin film transistor. Hence, the aperture ratio of eachpixel is increased to improve the display quality.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of various embodiments of the invention, and areincorporated in and constitute a part of this specification. Thedrawings illustrate embodiments of the invention and, together with thedescription, serve to explain certain inventive aspects of theinvention. In the drawings,

FIGS. 1A–1D are schematic, cross-sectional views showing a process forcontrolling the capacitance of the TFT-LCD storage capacitor accordingto one preferred embodiment of this invention;

FIGS. 2A–2C are schematic, cross-sectional views showing a process forcontrolling the capacitance of the TFT-LCD storage capacitor accordingto another preferred embodiment of this invention; and

FIGS. 3A–3C are schematic, cross-sectional views showing a process forcontrolling the capacitance of the TFT-LCD storage capacitor accordingto still another preferred embodiment of this invention.

DESCRIPTION OF CERTAIN EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

As described above, this invention provides a method of controlling thecapacitance of the TFT-LCD storage capacitor. This method controls thethickness uniformity of the storage capacitor's dielectric layer andthereby the effects of increasing uniformity of the storage capacitor'scapacitance, reducing the dimensions of TFT, and increasing the apertureratio of the LCD are reached.

Embodiment 1

FIGS. 1A–1D are schematic, cross-sectional views showing a process forcontrolling the capacitance of the TFT-LCD storage capacitor accordingto a first preferred embodiment of this invention. In FIG. 1A, a firstconductive layer is formed on a transparent substrate 100 and then ispatterned to form a gate 105 and a bottom electrode 110 respectively onthe transparent substrate 100. Then, a first silicon nitride layer 115,a dielectric layer 120, a second silicon nitride layer 125, an undopedamorphous silicon layer 130, and a doped amorphous silicon layer 135 aresequentially formed on the transparent substrate.

The material of the first conductive layer is, for example, copper,aluminum. chromium or alloy of molybdenum and tungsten, and the firstconductive layer can be formed by a physical vapor deposition processsuch as sputtering. The first silicon nitride layer 115 and the secondsilicon nitride layer 125 can be formed by chemical vapor deposition;the preferable thicknesses of the first silicon nitride layer 115 andthe second silicon nitride layer 125 are respectively about 1500–3500 Åand about 200–800 Å, and their more preferable thicknesses arerespectively about 2000–3000 Å and about 400–600 Å. An etchingselectivity ratio of amorphous silicon over the material of thedielectric layer 120 is not less than about 5.0. The material of thedielectric layer 120 is, for example, silicon oxide or dielectricmaterial, such as tantalum oxide, aluminum oxide or barium titanate,having a dielectric constant larger than about 4.0. The dielectric layer120 is formed by chemical vapor deposition; its preferable thickness isabout 100–600 Å, and its more preferable thickness is about 200–400 Å.

In FIG. 1B, the doped amorphous silicon layer 135, the undoped amorphoussilicon layer 130, and the second silicon nitride layer 125 arepatterned to form a stacked layer on the dielectric layer 120 over thegate 105. The stacked layer comprises the doped amorphous silicon layer135 a, the undoped amorphous silicon layer 130 a, and the second siliconnitride layer 125 a. The patterning method is, for example, lithographyand etching.

In FIG. 1C, a second conductive layer is formed on the transparentsubstrate 100. Then, the second conductive layer is patterned to formsource/drains 140 over both sides of the gate 105. Then, the dopedamorphous silicon layer 135 a exposed by opening 145 is etched to formlightly doped drains 135 b. The material of the second conductive layeris, for example, copper, aluminum, chromium or alloy of molybdenum andtungsten, and the second conductive layer is formed by a physical vapordeposition process such as sputtering.

In FIG. 1D, a passivation layer 150 is formed over the transparentsubstrate 100 and then is patterned to form a contact window 155 toexpose the source/drain 140 on the right side. A transparent conductivelayer is formed on the passivation layer 150 and in the contact window155. The transparent conductive layer then is patterned to form a pixelelectrode 160 to connect the exposed source/drain 140 through thecontact window 155 electrically. A storage capacitor of the thin filmtransistor is formed by the overlap between the pixel electrode 160 andthe bottom electrode 110, and hence the storage capacitor's dielectriclayer includes the passivation layer 150, the dielectric layer 120, andthe first silicon nitride layer 115 between the pixel electrode 160 andthe bottom electrode 110. The material of the above-mentionedtransparent conductive layer is, for example, indium tin oxide or indiumzinc oxide, and the transparent conductive layer is formed by, forexample, a physical vapor deposition process such as reactivesputtering.

Embodiment 2

FIGS. 2A–2C are schematic, cross-sectional views showing a process forcontrolling the capacitance of the TFT-LCD storage capacitor accordingto a second preferred embodiment of this invention. In FIG. 2A, a firstconductive layer is formed on a transparent substrate 200 and then ispatterned to form a gate 205 and a bottom electrode 210 on thetransparent substrate 200. A first silicon nitride layer 215, adielectric layer 220, a second silicon nitride layer 225, an undopedamorphous silicon layer 230, and an etching stop layer 235 aresequentially formed on the transparent substrate 200.

The material of the first conductive layer is, for example, copper,aluminum, chromium or alloy of molybdenum and tungsten, and the firstconductive layer is formed by a physical vapor deposition process suchas sputtering. The first silicon nitride layer 215 and the secondsilicon nitride layer 225 is formed by chemical vapor deposition; thepreferable thicknesses of the first silicon nitride layer 215 and thesecond silicon nitride layer 225 are respectively about 1500–3500 Å andabout 200–800 Å, and their more preferable thicknesses are respectivelyabout 2000–3000 Å and about 400–600 Å. An etching selectivity ratio ofamorphous silicon over the material of the dielectric layer 220 is notless than 5.0. The material of the dielectric layer 220 is, for example,silicon oxide or dielectric material, such as tantalum oxide, aluminumoxide or barium titanate, having a dielectric constant larger than about4.0. The dielectric layer 220 is formed by chemical vapor deposition;its preferable thickness is about 100–600 Å, and its more preferablethickness is about 200–400 Å. The etching stop layer 235 is formed bychemical vapor deposition, and it is, for example, a silicon nitridelayer or a silicon oxide/silicon nitride composite layer. The preferablethickness of the etching stop layer 235 is about 200–400 Å.

In FIG. 2B, the etching stop layer 235 is patterned to form an etchingmask 235 a on the undoped amorphous silicon layer 230 over the gate 205.Then, a doped amorphous silicon layer 240 and a second conductive layer245 are sequentially formed over the transparent substrate 200. Thematerial of the second conductive layer 245 is, for example, copper,aluminum, chromium or alloy of molybdenum and tungsten, and the secondconductive layer is formed by a physical vapor deposition process suchas sputtering.

In FIG. 2C, the second conductive layer 245, the doped amorphous siliconlayer 240, the undoped amorphous silicon layer 230, and the secondsilicon nitride layer 225 are sequentially patterned to form a stackedlayer and an opening 250 in the stacked layer. The stacked layercomprises a second silicon nitride layer 225 a, a channel 230 a, twolightly doped drain 240 a, and two source/drains 245 a. Since theetching mask 235 a protects the undoped amorphous silicon layer 230 a,the etching to form opening 250 can stop on the etching mask 235 a toavoid damaging the undoped amorphous silicon layer 230 a. The patterningmethod mentioned above is, for example, lithography and etching. Thefollowing processes are similar to those of Embodiment 1 and hence areomitted here.

Embodiment 3

FIGS. 3A–3C are schematic, cross-sectional views showing a process ofcontrolling the capacitance of the TFT-LCD storage capacitor accordingto a third preferred embodiment of this invention. In FIG. 3A, anundoped amorphous silicon layer is formed on a transparent substrate 300and then is patterned to form a silicon island 305 and a bottomelectrode 310 on the transparent substrate 300. A first silicon nitridelayer 315, a dielectric layer 320, a second silicon nitride layer 325,and a first conductive layer 330 are sequentially formed on thetransparent substrate 300.

The first silicon nitride layer 315 and the second silicon nitride layer325 is formed by chemical vapor deposition; the preferable thicknessesof the first silicon nitride layer 315 and the second silicon nitridelayer 325 are respectively about 1500–3500 Å and about 200–800 Å, andtheir more preferable thicknesses are respectively about 2000–3000 Å andabout 400–600 Å. An etching selectivity ratio of the material ofamorphous silicon over the dielectric layer 320 is not less than about5.0. The material of the dielectric layer 320 is, for example, siliconoxide or a dielectric material, such as tantalum oxide, aluminum oxideor barium titanate, having a dielectric constant larger than about 4.0.The dielectric layer 320 is formed by chemical vapor deposition; itspreferable thickness is about 100–600 Å, and its more preferablethickness is about 200–400 Å. The material of the first conductive layeris, for example, copper, aluminum, chromium or alloy of molybdenum andtungsten, and the first conductive layer is formed by a physical vapordeposition process such as sputtering.

In FIG. 3B, the first conductive layer 330 and the second siliconnitride layer 325 are patterned to form a stacked layer comprising asecond silicon nitride layer 325 a and a gate 330 a on the central partof the silicon island 305. Then, the gate 330 a is used as a mask toimplant ions into the silicon island 305 under both sides of the gate330 a and the bottom electrode 310 to form two source/drains 305 a,channel 305 b of the thin film transistor and the bottom electrode 310a.

In FIG. 3C, a passivation layer 335 is formed over the transparentsubstrate 300, and the passivation layer 335, the dielectric layer 320and the first silicon nitride layer 315 then are patterned to formcontact windows 340 and 350 to expose both of the source/drains 305 a.Next, a second conductive layer is formed over the transparent substrateand is patterned to form a data line 355 connecting the source/drain 305a on the left side through the contact window 350. A transparentconductive layer is formed over the transparent substrate 300. Thetransparent conductive layer is patterned to form a pixel electrode 345connecting the source/drain 305 a on the right side through the contactwindow 340. A storage capacitor of the thin film transistor is formed bythe overlap between the pixel electrode 345 and the bottom electrode 310a, and hence the storage capacitor's dielectric layer includes thepassivation layer 335, the dielectric layer 320, and the first siliconnitride layer 315 between the pixel electrode 345 and the bottomelectrode 310 a. The material of the above-mentioned transparentconductive layer is, for example, indium tin oxide or indium zinc oxide,and the transparent conductive layer is, for example, a physical vapordeposition process such as reactive sputtering.

From the preferred embodiments mentioned above, it is evident that thegate dielectric layer of the thin film transistor is replaced by thecomposite gate dielectric layer comprising the first silicon nitridelayer, the dielectric layer and the second silicon nitride layer in thisinvention. Therefore, when the undpoed amorphous silicon layer/dopedamorphous silicon layer in the bottom gate design are etched to form thestacked layer or the conductive layer in the top gate design is etchedto form the gate, the dielectric layer is used as an etching stop layer.Therefore, the thickness of the remaining gate dielectric layer over theentire transparent substrate is quite uniform after over-etching, andstorage capacitors with uniform capacitance on the entire transparentsubstrate is obtained after the subsequent steps of depositing thepassivation layer and forming the pixel electrodes.

The etching selectivity of an amorphous silicon layer over a siliconnitride layer is about 3.0–5.0, and the etching selectivity of anamorphous silicon layer over a silicon oxide layer is about 5.0–10.0. Ifthe gate dielectric layer being a silicon nitride layer compares withthe gate dielectric layer comprising a first silicon nitride layer, asilicon oxide layer and a second silicon nitride layer in a bottom gatedesigned thin film transistor, the result after over-etching is asfollows. In the case where the gate dielectric layer is a siliconnitride layer, the thickness uniformity is about 5% over entiretransparent substrate after depositing the silicon nitride layer bychemical vapor deposition; the thickness uniformity is reduced to about20% after over-etching. However, in the case where the gate dielectriclayer comprising a first silicon nitride layer, a silicon oxide layerand a second silicon nitride layer, the thickness uniformity is about 5%over the entire transparent substrate after depositing the first siliconnitride layer, the silicon oxide layer and the second silicon nitridelayer by chemical vapor deposition; the thickness uniformity is stillmaintained at about 5% after over-etching. In addition, the thicknessuniformity is also about 5% after depositing the passivation layer, andthe storage capacitor's capacitance is maintained at a quite gooduniformity over the entire transparent substrate.

Furthermore, if a dielectric layer with a dielectric constant largerthan 4.0 is used, the storage capacitor's capacitance is increased.Therefore, the dimensions of the thin film transistor is further reducedto increase the aperture ratio of the liquid crystal display to improvethe display quality.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A method of controlling a capacitance of a TFT-LCD storage capacitor, comprising: forming a silicon island and a bottom electrode on the transparent substrate, the silicon island having an undoped region located on the central portion, and two doped regions respectively located on both sides; forming a first silicon nitride layer on the silicon island, the bottom electrode and the transparent substrate; forming a dielectric layer on the first silicon nitride layer; forming a stacked layer comprising a second silicon nitride layer and a conductive layer on the dielectric layer above the undoped region of the silicon island, and the conductive layer of the stacked layer serving as a gate of a thin film transistor, wherein an etching selectivity ratio of the conductive layer over the dielectric layer is not less than about 5.0; forming a passivation layer over the stacked layer and the dielectric layer; forming a contact window in the passivation layer, the dielectric layer and the first silicon nitride layer to expose one doped region of the silicon island; and forming a pixel electrode on the passivation layer and in the contact window, and an overlap between the pixel electrode and the bottom electrode forming a storage capacitor.
 2. The method of claim 1, wherein the dielectric layer comprises a silicon oxide layer.
 3. The method of claim 1, wherein the dielectric constant of the dielectric layer is larger than 4.0.
 4. The method of claim 1, wherein the dielectric layer is one of a tantalum oxide layer, an aluminum oxide layer and a barium titanate layer.
 5. The method of claim 1, wherein the passivation layer comprises a silicon nitride layer.
 6. The method of claim 1, wherein the pixel electrode is made of indium tin oxide or indium zinc oxide.
 7. The method of claim 1, wherein the silicon island is made of amorphous silicon.
 8. Thee method of claim 1, wherein the two doped regions of the silicon island are formed by using the gate as a mask to implant ions into the silicon island under both sides of the gate.
 9. The method of claim 1, further comprising forming another contact window in the passivation layer, the dielectric layer and the first silicon nitride layer to expose another doped region of the silicon island, and forming a data line connecting the doped region of the silicon island through the contact window.
 10. The method of claim 1, wherein the two doped regions of the silicon island are respectively serving as a source and a drain of the thin film transistor. 